(1) Field
The present invention relates generally to vehicle airbags and, more particularly, to a coating that imparts high thermal resistance to an airbag's heat shield.
(2) Related Art
Airbags are used in the automotive industry to protect vehicle occupants in the event of an accident. An airbag system typically includes a sensor, inflator, airbag and cover. When the sensor is set off, for example in the event of an accident, the inflator is activated, and fills the airbag with gas. The airbag comes from behind (or through) the cover when inflated, thereby placing a cushion between the occupant and the potentially harmful vehicle surface. These events must occur within milliseconds to effectively protect the occupant.
Inflators are generally classified as cold-gas types, which release compressed gas, or pyrotechnic types, which burn a generant such as sodium azide or alkali metal azide. Combustion types are desirable because the generants' burn rates are controllable and reproducible, and therefore the devices are quite reliable. Combustion types also tend to be smaller, weigh less and be less expensive than compressed gas inflators.
Combustion inflators propel not only hot gas but also “clinkers,” which are by-product particulates such as sulfur metals and sodium azide. These clinkers are propelled at approximately 90 meters per second, at 450-1000° C. These projectiles can burn holes in the airbag, which is typically made of nylon or a similar fabric, and can lead to decreased airbag efficacy or burn injuries to the occupant. So called “filtered combustion inflators” attempt to prevent clinker-related problems by introducing a filter into the gas stream which inhibits clinkers from entering the airbag itself. However, filters are an imperfect solution given they slow the inflation rate, introduce another part into the system that could lead to failure, add weight and volume to the system, and cost more.
“Unfiltered combustion inflators” or “hot unfiltered inflators” are very desirable in the industry because they are reliable, small, light and relatively inexpensive. However, since they do not have filters, the clinkers spew out of the inflator with the inflating gas. Hot clinkers can compromise the integrity of an airbag by melting or burning through the airbag material, thereby causing under inflation or premature deflation.
Accordingly, various treatments and devices have been tried to lessen the negative impact of clinkers in airbags used with hot unfiltered inflators. Specifically, airbags have been treated with inside coatings of heat resistant material such as neoprene, silicone resin, silicone rubber, fluororesin such as PFA, various poly-resins, expanded graphite and intumescent material. Alternatively, liners have been employed using materials such as aluminized silica cloth and expandable fiberglass. Also, independent heat shields have been introduced made of materials such as stamped steel, aromatic polyamide fiber such as Kevlar®, as well as nylon, polyester and fiberglass coated with neoprene, silicone or poly-vinyl chloride. Finally, deflectors have also been made of the same material as the airbag.
While the aforementioned improvements are useful, they remain less than ideal. Specifically, airbags with a treated inside surface are still susceptible to burn-through by projectile clinkers because known coatings provide limited protection when applied thinly but a thickly applied coating defeats the safety system's overall goal of being lightweight and occupying a small volume. Liners are also problematic because they introduce more bulk and weight to the airbag. The independent shields and deflectors are likewise limited in that they are either unacceptably bulky or not bulky but provide only limited protection.
Thus, there remains a need for a new and improved vehicle safety device, airbag, heat shield and coating that exhibit improved thermal resistance values while, at the same time, is both low volume and low weight.